1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium in which a large amount of information can be recorded, and a method for recording the information.
Furthermore particularly, the present invention relates to an optical recording medium capable of pit length recording various kinds of information such as image, sound, code data and the like by irradiating the optical recording medium of a single plate type having a dye-containing recording layer with a semiconductor laser beam, a pit length recording method, and an optically pit length recorded medium recorded by means of pit length.
2. Description of the Related Art
In optical recording media having a dye-containing recording layer capable of writing by laser beam, the recording film can be formed by spin coating, and therefore, the media are excellent in productivity, economy and yield as compared with optical recording media having a recording layer composed of an inorganic thin film which is produced by vacuum technique. In particular, optical recording media having a phthalocyanine dye-containing recording layer are preferable since the media are excellent in durability.
Law, U.S. Pat. No. 4,492,750 discloses an optical recording medium using an alkyl substituted phthalocyanine dye as a recording layer. However, in order to protect the recording layer, a protective plate should be laminated on the recording layer with air gap. In the case of the form of the simplest medium structure of a single plate type which is so constituted that a thin film protective layer is directly provided on the phthalocyanine dye recording layer, the protective layer affects upon recording and therefore, it is difficult to form a recorded media with low error rate. In particular, it is more difficult to provide pit length recording media capable of giving a large amount of record.
Even if the recording is possible, the error rate and jitter value in each pit are so large that the reliability is poor. In addition, the optimum range of recording laser power is not wide upon recording.
Ozawa et al., U.S. Pat. No. 4,769,307, discloses an optical recording medium using an alkoxy substituted phthalocyanine dye. However, in this medium it is essential to bond two optical recording media with their recording layers facing inside with air gap to protect the recording layer.
When a metal reflective layer and a protective layer are directly formed on the recording layer of this medium to form a medium of a single plate type and a pit length recording is effected, error rate and jitter value of each pit are so large that the reliability is low even if the recording is possible.
Tanikawa et al, U.S. Pat. No. 4,458,004, discloses an optical recording medium comprising a recording layer containing a fluorine substituted phthalocyanine dye and a transparent protective layer directly provided on the recording layer. However, the fluorine substituted phthalocyanine dye has low solubility in solvents so that it can not be formed into a film directly overlying a thermoplastic resin substrate by a coating method. In addition, the medium using this dye can not give reflectivity of 60% or more even when a metal reflective layer is furnished. Further, when information is written to this media in the manner of pit length recording it is not possible to give a pit length recording of low error rate and low jitter value.
Nakagawa et al., Japanese Patent Application Laid-open No. 191690/1988, discloses an optical recording medium having a recording layer containing substituted naphthalocyanine dyes or substituted phthalocyanine dyes. However, in this medium two media are bonded with their recording layer facing inside with an air gap to protect the recording layer, but nothing is disclosed about a medium of a single plate type capable of pit length recording where a cover layer is positioned directly on a recording layer.
There also was proposed optical recording media comprising a recording layer composed of a dye, a metal reflective layer and a protective layer provided successively on the recording layer, which is reproduced with commercially available compact disk player (for example, Optical Data Storage 1989, Technical Digest Series, Vol. 1, 45 (1989), Japanese Patent Application Laid-open Nos. 132656/1990 and 168446/1990). Though these media have a high reflectivity, the light resistance and durability are poor due to the use of cyanine dyes, and moreover, when a pit length recording is effected, the pit jitter value for each length and jitter value of pits are not small, according to the present inventors' investigation.
Ito et al., EP-373,643, discloses a medium comprising a recording layer containing tetraalkoxyphthalocyanines, and a metal reflective layer directly provided on the recording layer. Though this medium exhibits a high reflectivity, pit jitter value for each length and jitter value of pits are not small when a pit length recording is carried out. Further, an optical recording card using tetra-alkoxy-tetrabromophthalocyanine dyes is also disclosed, but nothing is disclosed about an optical recording medium of a single plate type capable of pit length recording comprising a cover layer positioned directly on a recording layer.